Abstract
We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting Earth’s shadow, which is offset in opposite directions for opposite charges due to Earth’s magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 and 200 GeV. We confirm that the fraction rises with energy in the 20–100 GeV range. The three new spectral points between 100 and 200 GeV are consistent with a fraction that is continuing to rise with energy.
- Received 2 September 2011
DOI:https://doi.org/10.1103/PhysRevLett.108.011103
© 2012 American Physical Society
Synopsis
SynopsisEarth’s Magnetic Field Aids Study of Cosmic-Ray Puzzle
Published 5 January 2012
New spectra of cosmic-ray electrons and positrons measured by the Large Area Telescope, with the help of the Earth’s magnetic field, show a continuation of the puzzling increase in the fraction of positrons.
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